Neutrophils, the most abundant circulating white blood cells in the human body, patrol the capillary networks of the lung monitoring the airspace. Here they provide the first line of defense against invading pathogens and toxin exposures. While neutrophil migration is essential for pathogen elimination in the lung, inadvertent targeting of these cells to the airspace can result in life threatening lung damage. The long term goal of this research is to identify the molecular pathways utilized during TREM-1 (triggering receptor expressed on myeloid cells-1) dependent neutrophil migration into the airspace and leverage this information to develop therapeutic tools to prevent neutrophil-driven lung injury. TREM-1, initially discovered on human neutrophils and monocytes, serves as a critical amplifier of immune signaling. We recently discovered that neutrophil TREM-1 is required for transepithelial migration into the lung following gram negative infection. In addition to its established role in te amplification of inflammatory signaling, these findings identified a novel role for TREM-1 in neutrophil migration. During our characterization of human neutrophil migration, we identified a novel TREM-1 splice variant (TREM-1sv) protein stored in primary and secondary granules. This splice isoform is released during transepithelial migration. We also found in pilot studies that inhibition of extracellular ROS attenuates neutrophil transepithelial migration, and TREM-1 deficient neutrophils have reduced mitochondrial ROS production. In addition to a membrane isoform of TREM-1, a soluble form of the receptor has been described in human disease. However, the origin and function of this soluble molecule remain poorly understood. We hypothesize that neutrophil mitochondrial ROS and TREM-1sv modulate TREM-1 dependent transepithelial migration in the lung and that mitochondrial ROS is required for TREM-1 mediated migration. To test our hypothesis, we propose the following specific aims. 1. Determine if mitochondrial ROS is required for TREM-1 mediated neutrophil transepithelial migration. The goal of these studies is to investigate if pharmacologic and genetic modulation of mitochondrial ROS alters transepithelial migration. 2. Determine if TREM-1sv regulates ROS-driven neutrophil transepithelial migration. The goal of these studies is to determine if exogenous TREM-1 modulates the ability of neutrophils to cross the epithelial barrier either by altering neutrophil epithelial interactions or by abrogating mitochondrial ROS production. 3. Test the efficacy of TREM-1sv as a therapy to decrease neutrophil transepithelial migration in vivo. The goal of these studies is to determine if exogenous TREM-1sv can inhibit neutrophil migration into the airspace and decrease local inflammation.